Sulphate-reducing bacteria compete with methanogens for substrates in anaerobic digesters.
They reduce sulphate to hydrogen sulphide \((H_2S)\), leading to increased \(H_2S\) production and decreased methane yield.
Hence,
\[
\boxed{\text{Sulphate reducers}}
\]
is the correct answer.
Therefore,
\[
\boxed{(C)}
\]